September equinox

Today is the September equinox, a day when the Sun is above the horizon for exactly half the time everywhere on Earth. According to the astronomical definitions of the seasons, this day marks the beginning of autumn in the northern hemisphere and of spring in the southern hemisphere.

On the day of the equinox, the Sun will appear to rise from the point on the horizon which lies due east, and set beneath the point which lies due west. This happens as the Sun's annual journey across the sky, through the constellations of the zodiac, carries it across the celestial equator. As a result the Sun appears directly overhead at noon on the Earth's equator.

On the day of the equinox, the Earth's poles are not tilted towards or away from the Sun, meaning that the Sun is above the horizon for half the time at all latitudes. Image courtesy of Przemyslaw Idzkiewicz.

Equinoxes occur twice a year – in March and September – once when the Sun is travelling northwards, and once when it is travelling southwards. The position of the Sun at the moment of the March equinox is used to define the zero point of both right ascension and declination. At the September equinox, the Sun lies exactly opposite this point, at a right ascension of around 12 hours.

In practice this is not exactly the case, because of a phenomenon called the precession of the equinoxes. Over hundreds of years, the direction of the Earth's spin axis in space changes because it acts like a gyroscope. This means that the location of the equinoxes creep across the sky at a rate of around 50 arcseconds each year.

Astronomers quote right ascensions and declinations based on the configuration of the Earth's path around the Sun on January 1, 2000. Even in the years that have passed since the year 2000, the precession of the equinoxes has moved them by several arcminutes.

The table below lists the exact position of the Sun at the moment it passes over the equator in September 2018.